MOUNT_FDESC(8) BSD System Manager's Manual MOUNT_FDESC(8)
NAMEmount_fdesc -- mount the file-descriptor file system
SYNOPSISmount_fdesc [-ooptions] fdescmount_pointDESCRIPTION
The mount_fdesc command attaches an instance of the per-process file descriptor namespace to the global
filesystem namespace. The conventional mount point is /dev and the filesystem should be union mounted
in order to augment, rather than replace, the existing entries in /dev. This command is normally exe-cuted executed
cuted by mount(8) at boot time.
The options are as follows:
-o Options are specified with a -o flag followed by a comma separated string of options. See the
mount(8) man page for possible options and their meanings.
The contents of the mount point are fd, stderr, stdin, stdout and tty.
fd is a directory whose contents appear as a list of numbered files which correspond to the open files
of the process reading the directory. The files /dev/fd/_ through /dev/fd/# refer to file descriptors
which can be accessed through the file system. If the file descriptor is open and the mode the file is
being opened with is a subset of the mode of the existing descriptor, the call:
fd = open("/dev/fd/0", mode);
and the call:
fd = fcntl(0, F_DUPFD, 0);
are equivalent.
The files /dev/stdin, /dev/stdout and /dev/stderr appear as symlinks to the relevant entry in the
/dev/fd sub-directory. Opening them is equivalent to the following calls:
fd = fcntl(STDIN_FILENO, F_DUPFD, 0);
fd = fcntl(STDOUT_FILENO, F_DUPFD, 0);
fd = fcntl(STDERR_FILENO, F_DUPFD, 0);
Flags to the open(2) call other than O_RDONLY, O_WRONLY and O_RDWR are ignored.
The /dev/tty entry is an indirect reference to the current process's controlling terminal. It appears
as a named pipe (FIFO) but behaves in exactly the same way as the real controlling terminal device.
FILES
/dev/fd/#
/dev/stdin
/dev/stdout
/dev/stderr
/dev/tty
SEEALSOmount(2), unmount(2), tty(4), fstab(5), mount(8)CAVEATS
No ~. and .. entries appear when listing the contents of the /dev/fd directory. This makes sense in
the context of this filesystem, but is inconsistent with usual filesystem conventions. However, it is
still possible to refer to both ~. and .. in a pathname.
This filesystem may not be NFS-exported.
HISTORY
The mount_fdesc utility first appeared in 4.4BSD.
4.4BSD March 27, 1994 4.4BSD

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